1. Field of the Invention
The present invention relates to a silver halide emulsion suitable for preparing a photosensitive emulsion layer to be incorporated in silver halide photographic paper. More particularly, the invention relates to a silver halide emulsion that ensures improved stability in photographic properties against variations in the concentrations of bromide ions in developing solutions and which provides an emulsion coating with improved longterm stability.
2. Description of the Prior Art
In order to meet the recent demand for rapid processing of silver halide photographic materials, most modern large-scale laboratories use automatic processors that enable running processing. However, one big problem with this system is that the composition of the processing solution and process conditions vary from one laboratory to another and even within the same laboratory, and as a result, the photographic characteristics of the materials being processed fluctuate so greatly that products of consistent performance cannot be obtained. It is generally understood that this variation in the composition of the processing solution and the process conditions is caused mainly due to the dissolution of photographically active materials out of the photographic material under running processing and their accumulation in the processing solution. In order to prevent the variation in the process conditions, it is necessary to effect a high degree of control over the development period, the temperature of the developing solution, its pH, and the concentration of halide ions, particularly, bromide ions in the developing solution. However, the concentration of bromide ions in the developing solution is more difficult to control than the development period and the temperature of the developing solution because their concentration defies a very accurate determination.
It is therefore necessary to obtain a highly stable silver halide emulsion the photographic characteristics of which are less dependent on the concentration of bromide ions. Needless to say, this stability should not be achieved by sacrificing the most basic properties of photographic materials, namely, speed (sensitivity), fog resistance and gradation in tone. Among other things, silver halide emulsions for use in photographic paper require particularly rapid development, and inevitably, they contain chloride in the silver halide. In this case, the requirements to be met are high gradation and low fog rather than high speed.
Hydroxytetrazaindene compounds and mercapto compounds are conventionally known as stabilizers, antifoggants or desensitizers for photographic emulsions. A detailed discussion of hydroxytetrazaindene compounds is found in V. C. Chambers, "A Corelation of the Chemical Structures of Some Triazolo Pigments with their Photographic Effects" in "Photographic Science & Engineering", vol. 6, No. 2. Details of mercapto compounds are given in, for example, "The Thory of the Photographic Process", 3rd ed. edited by C. E. Kenneth Mees and T. H. James, The Macmillan Company, chapter 15, pp. 344-346.
It is also known that hydroxytetrazaindene or mercapto compounds can function as sensitizers within emulsions. For example, Japanese patent application (OPI) No. 63914/75 (the symbol OPI as used herein means an unexamined published Japanese patent application) and West German patent application (OLS) No. 2,419,798 show that a higher sensitivity can be attained by incorporating hydroxytetrazaindene compounds in sulfur-sensitized monodisperse silver halide emulsions comprising cubic grains with a silver bromide content of 80 mol % or more. Japanese patent application (OPI) No. 36130/76 show that a higher sensitivity can also be achieved by combining a mercaptocontaining heterocyclic compound having 2 or more nitrogen atoms and a PKa of 7.6 or less with sulfur-sensitized silver halide grains in cubic form having a (100) plane and containing at least 80 mol % of silver bromide or chloride. There are many other prior art references that show the incorporation in emulsions of either hydroxytetrazaindene compounds or mercapto compounds or both. However, no prior art references have shown techniques for providing stability against variations in the concentration of bromide ions in the developing solution. There is also no prior art teaching of combining an emulsion of highly monodispersity with both a hydroxytetrazaindene compound and a nitrogen-containing heterocyclic compound having at least one mercapto group.
In the development of common silver halide emulsions, desensitization and change in gradation occur as the concentration of bromide ions in the developing solution increases. The most immediate cause of these phenomena would be the decrease in the electric potential of the developing solution due to the increasing concentration of bromide ions. Other factors would be the interaction between bromide ions and the developing nuclei, and the conversion of other halide ions to bromide ions. For whichever reason, these factors determine the developing speed, and the desensitization or the change in gradation may well be explained by the change in developing speed.
It is common sense in the art that the increase in the size of silver halide grains is accompanied by higher sensitivity and lower developing speed. Silver halide emulsions comprising small grains and having high chlorine content, for example, those having fairly high developing speed such as silver chlorobromide or chlorobromoiodide emulsions for photographic paper, have a tendency to desensitize and provide a harder tone as the concentration of bromide ions in the developing solution increases. This means that the toe of the photographic characteristic curve suffers a great degree of desensitization whereas the shoulder experiences only a small degree of desensitization.
When silver halide grains do not have a high degree of monodispersity, the sensitivity of toe region in the characteristic curve depends on larger grains. Therefore, the above described tendency toward hard tone that accompanies the increasing concentration of bromide ions in the developing solution seems to be attributable to the greater inhibition of development by larger grains. Based on this assumption, the present inventors expected that the variation in the toe sensitivity would be reduced to a level comparable with the variation in the shoulder sensitivity by using silver halide grains of high monodispersity. However, as it turned out, the desired stability could not be obtained by simply using silver halide grains having a high degree of monodispersity.